I. Field of the Invention
The invention is related to electrical switches and in particular to a photoelectric switch for initiating or changing a mode of operation of a machine in a user safe manner.
II. Description of the Prior Art
It has long been known to use a pair of spatially separated manually operable switches to activate the operational modes of machines such as stamping presses, riveters, tube benders, spot welders or like machines. The purpose of the spatially separated manually operated switches is to ensure that both of the operator's hands are positioned away from the machine prior to its operation. To further ensure the safety of the operator, the pair of switches must be activated simultaneously and must be placed apart a distance sufficient to prevent simultaneous operation of both switches by a single hand. The Federal Occupational Safety and Health Administration (OSHA) has set the criteria that must be met by such machine operating equipment to ensure adequate operator safety.
The prior art initially used a mechanically actuated single pole double throw palm button switch. These palm button switches are biased to a normally open position and constant depression is required to close the normally open contact. The use of a pair of spatially separated palm buttons has been adequate for the intended safety purposes. However, because the operator must apply significant pressure to the palm button switches in order to achieve actuation, the operators are experiencing inflammation of the tendons due to the repetitive and forceful action of the fingers and wrists. This inflammation of the tendons of the fingers and wrists is known as Carpal Tunnel Syndrome. This ailment has increased worker fatigue and complaints, has decreased productivity and has increased the frequency of disability claims under workmen's compensation laws and programs.
The problems associated with the palm button switch has been substantially eliminated by the photoelectric switch taught by Herman et al in U.S. Pat. No. 4,939,358. This switch apparatus employs an infrared light source and a photodiode placed on opposite sides of a slot designed to receive one or more fingers of the operator. The fingers of the operator, when in the slot, occlude or break the infrared light path between the light source and the photodiode. The photodiode, in response to breaking or occlusion of the infrared beam, actuates a relay to switch its pole from a normally closed contact to a normally open contact. Conversely, when the infrared beam is restored, the relay is deactivated and the pole of the relay returns to the normally closed contact. Although the photoelectric switch taught by Herman et al resolved most of the problems associated with the palm button switches, the insertion of the fingers into the slot has been found to be uncomfortable for the operator and the sides of the fingers became sore when they are repeatedly banged against the bottom of the slot.
The invention is an improved photoelectric switch which is user friendly and incorporates a plurality of safety features not normally provided by the prior art switches.